When supplying electric power generated in an electric power station, or the like, there are used a transformer for reducing a voltage, a distribution board for distributing the electric power, and the like. The transformer, the distribution board, and the like each use a device for receiving and distributing the electric power, a controlling device such as a switch, and the like, in order to receive and distribute the electric power at a large capacity and a low voltage. In addition, the device for receiving and distributing the electric power, the controlling device, and the like each use a bus bar or a conductive member called a bus duct, in which a plurality of such bus bars are stacked (for example, Patent Literature 1).
For the conductive member, a copper-based material formed of copper or a copper alloy is mainly used because the copper-based material exhibits excellent performance in conductivity, strength, processability, corrosion resistance, and the like. However, in recent years, copper has increased in price owing to, for example, concern about depletion of copper resources. In addition, by its nature, the copper-based material is heavy in weight owing to, for example, copper having a density of 8.95 g/cm3 (20° C.) as compared to an aluminum material formed of aluminum or an aluminum alloy (for example, pure aluminum has a density of 2.699 g/cm3 (20° C.)). For those and other reasons, in all electricity-related fields, the aluminum material, which has a light weight, is easy to handle, and has excellent conductivity, has begun to attract attention as an alternative to the copper-based material.
However, a highly reactive metal such as aluminum has a property of being easily oxidized in its surface. For example, when the aluminum material is exposed to external air, its surface is immediately oxidized and a natural oxide film (aluminum oxide) is formed. Further, in an aluminum material subjected to hot plastic processing steps such as rolling, extrusion, and forging, a relatively thick and stiff thermal oxide film is formed on its surface. In the case where conductive members are manufactured through use of such aluminum material, electric resistance increases due to the oxide film formed on the surface to inhibit conductivity, and a problem of heat generation occurs particularly in a connecting section between the conductive members when a large-capacity current flows. Further, when the conductive member having the oxide film formed thereon is left in a high-temperature and high-humidity environment, the thickness of the oxide film gradually increases, and the oxide film and moisture react with each other to form a hydrate (hydrate film); with the result that electric resistance increases with time to cause trouble in an application of the conductive member.
Therefore, in a worksite where the conductive member made of the aluminum material is used, for example, when the conductive member is joined to a terminal serving as a joining target, or the like, an oxide film formed in a joining region of the conductive member is removed with a wire brush, or the like immediately before operations. Then, a conductive auxiliary coating agent, for example, conductive auxiliary grease obtained by mixing conductive auxiliary powder such as chromium oxide into grease is applied to the joining region of the conductive member, and the conductive member is joined to another conductive member through intermediation of the conductive auxiliary grease (Patent Literature 2). However, when all the operations are performed at the worksite, there is a problem in that not only operation efficiency but also operation quality is degraded, with the result that the quality of a conductive member to be obtained is degraded. That is, it was difficult to uniformly remove the oxide film and quantitatively manage the removed state of the oxide film at the worksite. In particular, the thermal oxide film was thick and stiff, and hence it was difficult to remove the thermal oxide film. Further, the surface roughness of the joining region was liable to increase. Further, in the application operation of the conductive auxiliary grease performed after the removing operation of the oxide film, it was also difficult to uniformly apply the conductive auxiliary grease and it was not even possible to quantitatively manage the application amount of the conductive auxiliary grease. In order to solve the above-mentioned problems, it has been considered to form a conductive member having the conductive auxiliary coating agent such as conductive auxiliary grease applied thereto in advance.
Meanwhile, the conductive auxiliary coating agent contains insulating grease as a main component, and hence contact resistance increases when another conductive member serving as a joining target or the like is joined to the conductive auxiliary coating agent in the case where the application thickness thereof is large. Therefore, in the case where a conductive member is joined to another conductive member or the like through intermediation of the conductive auxiliary coating agent, the following measures are frequently taken. Specifically, a contact surface pressure of the joining is increased to firmly join the conductive members to each other so that the conductive auxiliary coating agent is discharged properly from between the joined members to decrease the thickness of the conductive auxiliary coating agent.
However, when the contact surface pressure is increased, in the case where the strength of the conductive member or a fastening bolt is insufficient, there is a risk in that the buckling or deformation of the conductive member, the fracturing of the bolt, or the like may occur. In order to reduce the contact resistance by increasing the contact surface pressure without causing the above-mentioned problems, it is necessary to increase the contact surface pressure by enhancing a fastening pressure through use of bolts and nuts having a large fastening torque and increasing the number of bolts and nuts. Therefore, it is difficult to apply the conductive auxiliary coating agent to a small conductive member. That is, in the case of applying the conductive auxiliary coating agent to a conductive member to be used as, for example, a small bus bar for an automobile or the like, it is necessary to reduce the contact resistance by decreasing the thickness of a conductive-auxiliary-coating-agent layer after fastening even in a fastening torque of from 2 N·m to 10 N·m in the case of using bolts and nuts with a small diameter having a relatively low fastening toque, for example, in the case of using an air-driven or electric impact wrench. However, in the case where the surface roughness of the surface of a conductive base material is large, when the thickness of the conductive auxiliary coating agent is small, a gap through which oxygen and moisture enter remains in a fastening section. In this case, there is a risk in that the absolute amount of the conductive auxiliary coating agent may become insufficient, and oxygen and the like may enter a fastening surface of the conductive member to cause oxidation of the fastening surface to proceed, with the result that the contact resistance of the conductive member increases with time to make it impossible to keep sufficient conductivity.
Further, in the case where the conductive member having the conductive auxiliary coating agent applied thereto as described above is used in such a manner as to be stored or distributed as it is, there is a risk in that the applied conductive auxiliary coating agent may be lost or contaminated to become unsuitable for use due to contact with another object, the adhesion of foreign matters such as grit and dust to the conductive auxiliary coating agent, or the like. Further, there is also another problem in that the conductive auxiliary coating agent may contaminate another object that the conductive auxiliary coating agent is brought into contact.